1. Field of the Invention
The present invention concerns a metal base circuit board for use in fields where heat dissipation and thinness are demanded, such as liquid crystal display devices that use LEDs (light emitting diodes) as light sources.
2. Description of the Related Art
Examples of light sources for liquid crystal backlights are CCFLs (Cold Cathode Fluorescent Lamps) and LEDs.
CCFLs have ease of use and long operating lifespan as advantages, whereas LEDs have the advantages of excellent color reproduction, high brightness, and thinness, and since they do not use mercury they have a lower environmental impact in comparison with CCFLs, and they are resistant to vibration and impact, and can be used over a large temperature range (−40 degrees C. to 85 degrees C.), and further, can have an operating lifespan of 50 thousand hours depending upon the operating environment.
Since the luminous efficiency of LEDs is low in comparison with CCFLs, in enclosed spaces such as the interior of liquid crystal backlights, there is a tendency for the temperature in the vicinity of LEDs to be higher in comparison with CCFLs. Under such conditions, the brightness decreases, and because it is then necessary to increase the electric current, the amount of heat generated increases, and the temperature rises further.
Because of this, the deterioration of the semiconductor elements is accelerated due to the temperature rise of the LEDs, and the operating lifespan of the LEDs becomes shorter. Because of this, printed circuit boards on which LEDs are mounted demand good heat dissipation.
As one method for dissipating heat in LED-mounted printed circuit boards, there is a method whereby printed circuit boards release heat to the housing via a heat conducting material (see patent document 1). However, in this method, it is difficult for a printed circuit board alone to dissipate heat to the housing, and accessory members become necessary, so the cost increases, and this is undesirable.
In another heat releasing method, there is a structure wherein heat is released from a heat releasing portion that is also a circuit pattern for a printed circuit board, through thermal vias to a heat releasing pattern on the reverse side, and then to the housing (see patent document 2). However, there is the problem that the constitution of the board is complex.    Patent Document 1: JP-A 2002-353388    Patent Document 2: JP-A 2007-12856